The invention relates to an assembly and to a method for storing energy in electrochemical cells having large diameters and high storage capacity. A preferred field of application of the invention is to provide energy balancing.
Reconciling the supply and demand in the provision of electric energy poses a major problem in energy management, which is further exacerbated by the intensified use of forms of renewable energy that fluctuate over time, such as wind energy and solar energy.
A variety of forms for storing energy have been proposed so as to assure the necessary daily and seasonal equalization. Traditionally, pumped storage power stations haven been utilized for this purpose, but compressed air energy storage and molten salts are meanwhile being used as well.
Electrochemical energy stores can also be used for this purpose. Since the introduction of lead storage batteries, a large number of additional systems have been created, only a relatively small portion of which has become established for practical use. The useful life, which is to say the maximum number of cycles, of storage batteries containing solid electrodes is generally limited by aging-induced changes of the electrode structures. This problem can be counteracted by using liquid electrode materials. Examples of storage batteries containing liquid electrodes include rechargeable sodium-sulfur (NaS) and sodium-nickel chloride (ZEBRA) batteries. In both systems, the electrolyte is a β″-Al2O3 ceramic, which conducts sodium ions at higher temperatures (solid electrolyte). This ceramic material is brittle and sensitive toward thermal shocks, which in practical experience limits the usable diameter of individual cells to a few centimeters. Approximately 20,000 individual cells must typically be interconnected in order to output power of 1 MW. So as not to mechanically stress the solid electrolyte, the cells should be maintained at the operating temperature (˜300° C.) to as great an extent as possible over the entire useful life to ensure that the electrode materials remain liquid, and this leads to the corresponding thermal losses.
U.S. 2008044725 A 2008-02-21 proposed an electrochemical energy storage unit by which these drawbacks are supposed to be overcome. Here, metalloids or metals such as arsenic, antimony, bismuth, selenium, or tellurium, which have sufficiently high electrical conductivity in the liquid state, function as the positive electrode (conventionally designated as the “cathode” during discharging). Magnesium, potassium, sodium, lithium, calcium, cadmium, and zinc are proposed for the negative electrode (anode). An important aspect is the use of a liquid electrolyte between the anode and cathode. The materials are selected so that the density of the electrolyte is higher than that of the liquid anode material, and the density of the liquid cathode material is higher than the density of the electrolyte. Under these circumstances, natural density stratification of the materials takes place, whereby the necessity of using a porous membrane is dispensed with. This option was incidentally already indicated in U.S. Pat. No. 3,245,836 A.
The typical current density j, which can reportedly be achieved with such an assembly, is indicated in U.S. 2008044725 A 2008-02-21 as a value of 10 to 50 kA/m2, with assemblies having greater current densities also being claimed.
A key drawback of the proposed assembly is the limitation of the maximum current of the cell resulting from physical principles. The arbitrary scalability claimed in U.S. 2008044725 A 2008-02-21 of quantities of several cubic meters (quote: “In one approach, scalability is exploited in a single large cell of the invention—on the order of a few meters cubed . . . ”) cannot be achieved in reality. This is due to the fact that, for a given current density, the maximum diameter of the cell is limited by the onset of a known current instability in liquid conductors. This instability was first described in 1972 by Vandakurov, VANDAKUROV, Y. V., Theory for the stability of a star with toroidal magnetic field, Soviet Astronomy, 1972, Volume 16, pages 265-272 and in 1973 by Tayler, TAYLER, R. J., Adiabatic stability of stars containing magnetic fields. 1—Torodal fields. Monthly Notices of the Royal Astronomical Society, 1973, Volume 161, pages 365-380 and calculated in detail for the case of cylindrical assemblies by Rüdiger et al, RÜDIGER, G., Theory of current-driven instability experiments in magnetic Taylor-Couette flows, Physical Review E76, 2007, page 053309; RÜDIGER, G., Tayler instability of toroidal magnetic fields in MHD Taylor-Couette flows. Astronomische Nachrichten [Astronomy News] 2010, no. 331, pages 121-129. This instability, which hereafter is referred to as the Tayler instability, causes electric currents to become unstable in liquid metals if they exceed several kA (depending on the specific substance parameters). In connection with this instability, a very strong flow within the liquid metal occurs, which would result in strong intermixing of anodic, electrolytic, and cathodic materials, and thereby render the assembly unusable as an energy storage unit.